In a mobile communication system using a cellular system, communication quality normally becomes worst in the vicinity of the boundary between cells. For this reason, in the case where the mobile station is positioned in the vicinity of the boundary between cells, signals received by a plurality of base stations are combined or the signal having the best quality is selected from among the signals received by a plurality of base stations. This makes it possible to restrain deterioration of communication quality of a reverse link (link directing from the mobile station to the base station).
The receiving method in which the signals received by the plurality of base stations are combined to restrain the deterioration of the reception quality is hereinafter referred to as combining-type reception. While, the receiving method in which the signal having the best reception quality is selected from among the signals received by the plurality of base stations to restrain the deterioration of the reception quality is referred to as selecting-type reception.
Moreover, in the CDMA mobile communication system, transmission power control is performed in order to solve the so-called near-far problem.
In the case where the mobile station performs the so-called open-loop type transmission power control in connection with transmission power control, the mobile station subtracts a power value of a received signal from a transmission power value informed from the base station in advance so as to measure a propagation loss between the base station and the mobile station. Then, the mobile station adds the value of propagation loss to a desired reception power value at the base station so as to determine a transmission power value.
However, the following problem exists in the case where the system that performs the combining-type reception and the system that performs selecting-type reception are mixed.
Namely, the system that performs the combining-type reception can increase reception power by an amount corresponding to a diversity gain as compared with the system that performs selecting-type reception. Accordingly, if the mobile station directly performs transmission power control of open-loop type, which is carried out in the system that performs selecting-type reception by the mobile station, in the system that performs the combining-type reception, the reception quality exceeds a desired quality greatly at the base station. This results in poor efficiency in view of the system capacity.
For this reason, it is desirable that the mobile station should change the transmission power control method appropriately according to the reception type of the system where the mobile station is currently positioned. However, such a method has not been disclosed so far.